A duct system, especially an intake manifold for an internal combustion engine, having flap valves (13) for selective opening and closing of at least some of the ducts. The valves should be continuously adjustable in position in order to produce a swirl in the combustion airflow. To achieve this a high degree of angular precision is required in the positioning of the valves. Intake manifold manufacturing tolerances need to be compensated during assembly of the valves. Valve adjusting mechanisms are provided on the axles of the valves or in the mechanical connections of the valve gear pertaining to the valve drive mechanism.